1. Field of the Invention
This invention relates to a grating optical fiber, an optical fiber base material suitable for grating, and method for manufacturing the grating optical fiber.
2. Description of the Related Art
Zero-dispersion fibers (SMF) for a wavelength of 1.3 xcexcm and zero-dispersion fibers (DSF) for a wavelength of 1.55 xcexcm are widely used as transfer optical fibers. If a periodic change (such as a diffraction grating) is provided to the transfer optical fiber, reflection or optical coupling occurs with respect to a specific wavelength. Based on this fact, many studies and researches for forming a grating on an optical fiber in order to extract a specific wavelength have been made.
If the optical fiber is irradiated by ultra-violet rays via a mask having a transparent pattern, the index of refraction of the irradiated portion differs from that of the blocked portion of the optical fiber. By making use of this phenomenon, a grating can be written on the optical fiber. In particular, if the germanium doped in the quartz glass of the optical fiber is irradiated by ultra-violet rays, the lattice defect increases, and therefore, the index of refraction of the portion exposed to the ultra-violet rays increases. Since gratings can be written only in the areas in which germanium is doped, a grating can be formed on the core, which contains doped germanium, when the optical fiber is irradiated by ultra-violet rays.
FIG. 1 is a cross-sectional view of a conventional grating optical fiber taken along the longitudinal axis. This figure also shows the grating writing area 56 and the light propagation range 55. As shown in FIG. 1, light propagates not only through the core, but it slightly spreads out of the core into the clad 54. Even if the optical fiber has a grating on the core which has the filter effect for reflecting a certain light component having a specific wavelength, the filter property is likely to deteriorate because of the leakage of the light into the clad 54.
Therefore, it is an object of the invention to overcome the above-described drawback in the prior art, and to provide a grating optical fiber and an optical fiber base material suitable for grating which has a superior filtering ability.
In order to achieve the object, in one aspect of the invention, a grating optical fiber comprises a core, a first clad layer surrounding the core, and a second clad layer surrounding the first clad layer, and gratings are formed in both the core and the first clad layer.
Germanium is doped in the core, while germanium and fluorine are doped in the first clad layer.
Preferably, the difference between the specific indexes of refraction of the first and second clad layers is smaller than the difference between the specific indexes of refraction of the core and the first clad layer.
Preferably, the ratio of the outer diameter of the core to the outer diameter of the first clad is about 1:2 to 1:6.
In another aspect of the invention, an optical fiber base material suitable for grating is provided. The optical fiber base material comprising a core material which forms the core of an optical fiber, a first clad layer surrounding the core material, a second clad surrounding the first clad. Germanium is doped in the core material, and germanium and fluorine are doped in the first clad layer.
In still another aspect of the invention, a method for manufacturing an optical fiber base material which is used as a base material of an optical fiber, is provided. This method comprises the steps of preparing a core in which germanium is doped; forming a first clad layer, in which germanium and fluorine are doped, around the core; and forming a second clad layer around the first clad layer.
The first clad layer is formed by preparing a quartz axis; depositing silica soot, in which germanium is doped, on the quartz axis to form a silica soot layer around the quartz axis; removing the quartz axis from the silica soot layer; sintering the silica soot layer in the fluorine atmosphere to form a sintering glass shell; etching and polishing the sintering glass shell to form a polished glass shell; and inserting the core into the polished glass shell.
The second clad layer is formed by preparing a quartz axis; depositing silica soot on the quartz axis to form a second silica soot layer; removing the quartz axis from the second silica soot layer; sintering the silica soot layer in the fluorine atmosphere to form a second sintering glass shell; etching and polishing the sintering glass shell to form a second polished glass shell; and inserting the first clad layer into the polished glass shell.
Alternatively, the first clad layer may be formed by preparing a plurality of first clad sticks in which germanium is doped; and bundling the plurality of first clad sticks around the core to form the first clad base. In this case, the second clad layer is formed by forming a tube-like second clad layer shell; inserting the first clad base into the second clad layer; and jacketing the first clad base and the second clad layer into a single unit.
Preferably, the maximum number of the first clad sticks that can be inserted in the second clad layer shell are inserted in the second clad layer shell so as to surround the core inside the second clad layer shell.
In still another aspect of the invention, a method for fabricating a grating optical fiber from an optical fiber base material is provided. Such an optical fiber base material comprises a core in which germanium is doped, a first clad layer in which germanium and fluorine are doped, the first clad layer being formed around the core, and a second clad layer formed around the first clad layer. This method comprises the steps of wiredrawing the optical fiber base material to form an optical fiber; and irradiating the optical fiber to ultraviolet rays to write gratings on the core and the first clad layer.